The present system and method relates to maskless photolithography also called direct-write digital image technology for an ultra-large size flat panel display (FPD) patterning, and more particularly, it relates to an exposure apparatus for projecting a pattern directly onto an ultra-large substrate from a computer system so that the pattern is aligned with a previously formed substrate pattern in the computer system to produce an ultra-large flat panel display and the like.
Flat panel display (FPD) substrates have widely been used as display elements for personal computers, television sets and the like. Typically, a liquid crystal display (LCD) substrate is manufactured by forming transparent thin film electrodes on a photosensitive substrate (glass substrate) by photolithography. To carry out the photolithography, projection exposure apparatuses project a mask pattern onto a photoresist layer formed on a glass substrate through a projection optical system.
Recently, it has been desired that the area of flat panel display substrate be increased, and, accordingly, to increase an exposure area of the projection exposure apparatus.
In manufacturing large thin-film transistor LCDs, mass-producing 6 or 8 panels on a single glass substrate is typically most efficient. As demand for larger and larger LCDs continues to grow, manufacturers have increased mother glass dimensions from 680×880 mm 10 years ago to up to 2880×3080 mm now in mass production. There are several companies who are building the 10th Generation factory, which uses 2880×3080-mm glass.
In order to increase the exposure area, there has been proposed an exposure apparatus of so-called step-and-scan. In step and scan, after an initial exposure, the mask and the photosensitive substrate are shifted by a predetermined distance in the direction perpendicular to the scanning direction and then another scanning-type exposure is achieved.
The pixel cell array and color filter patterning processes in LCD manufacturing create some of the greatest challenges in scaling to Gen 10, in terms of both technology requirements and manufacturing costs. Typical alpha silicon (a-Si) thin-film transistors have critical dimensions around 3.5 μm and require alignment accuracy of ±1 μm. In color filter manufacturing, only the black matrix step (a black screen like pattern formed on the color filter that prevents light leakage, improves contrast and separates RGB sub-pixels) requires less than 10 μm resolution and alignment accuracy of less than ±3 μm, RCSB pixels, spacers and vertical alignment protrusions typically do not necessitate resolution precision of less than 20 μm. Even though LCD exposure specifications are large compared with those of semiconductors, the challenges in exposing very large areas and maintaining throughput presents serious issues to be overcome.
The primary method of maintaining productivity as substrates have grown has been to increase the size of the mask and exposure field. The largest masks used in production today for Gen 8 are 1220×1400×13 mm. With a pellicle (a thin, transparent membrane that prevents particles from contaminating the mask surface) attached, these easily can cost more than $350,000 for a single binary mask. To maintain throughput at Gen 10 and expose 2880×3080 mm substrates in four scans, photolithography and mask companies are developing even larger masks in the range of 1600×1800×17 mm. Initially, these very heavy quartz masks may cost more than $1 million apiece.
In the case of array exposure, average Gen-10 machine prices are expected to be nearly six times higher than those of Gen-4 machines, white the average increase for other tool types likely will be around twice as high. In 2000, photolithography costs accounted for only 14 percent of total array equipment spending, but when Gen-10 tools begin shipping, the costs are expected to account for up to 29 percent. For these reasons, exposure is a prime target of cost-cutting strategies.
In conventional photolithography, the patterned masks or films for high resolution application are typically very expensive and have a short lifetime. In addition, the photomasks are characterized as requiring a very long lead time. The long mask lead time is a problem when a short product development cycle is desired. Further, if a particular mask design is found to require a design change, no matter how small the change, then mask modification cost and lead time to implement the required change can cause serious manufacturing problems.
At present there is a need for a viable alternative to conventional photolithography for mass production that can meet all of the requirements of the pixel cell array process in LCD manufacturing.